The present invention relates to two-phase electric step motors, and in particular relates to details of the stator coil-driver connections for maximizing torque or other motor performance characteristics at a given pulse rate or step speed.
Two-phase bipolar step motors are widely used for motion control applications. FIGS. 1A and 1B show a typical eight-pole stator of a two-phase step motor of the prior art. Mechanically, the motor (not shown) appears in actual implementation as a multi-tooth gear, and the stator poles have corresponding teeth that mesh with the rotor so as to permit many motor steps per revolution. The step resolution (the number of steps per revolution) of a step motor is determined by multiplying the number of rotor teeth by the number of mechanical phases. The number of mechanical phases are two-times the number of electrical phases in a bipolar device.
Electrically, four stator coils (SC1, SC2, SC3 and SC4) are wound around the eight stator poles in a specified manner. A bifilar winding pattern is used, meaning that the stator coils are wound around the poles in pairs. Thus, first and second stator coils SC1 and SC2 are paired throughout the winding, and likewise the third and fourth stator coils SC3 and SC4 are paired throughout the winding. Each stator coil is wound around every other stator pole in alternating clockwise and counterclockwise directions. The ends of the stator coil wires are designated a1 and a1xe2x80x2 for SC1, a2 and a2xe2x80x2 for SC2, b1 and b1xe2x80x2 for SC3, and b2 and b2xe2x80x2 for SC4. These coil ends can be connected together and to a driver circuit in any of several different ways.
FIG. 1C shows a series connection for the stator coils of FIGS. 1A and 1B. The end a1xe2x80x2 of first stator coil SC1 connects to the end a2 of second stator coil SC2, so that coils SC3 and SC4 are also connected in series. Ends a1 and a2xe2x80x2 form terminals connecting coils SC1 and SC2 to phase A of a two-phase driver. A reversed current through coils SC1 and SC2 having a 180xc2x0 phase shift from phase A is designated as phase {overscore (A)}. Ends b1 and b2xe2x80x2 form terminals connecting coils SC3 and SC4 to phase B of the two-phase driver, wherein phases A and B have a 90xc2x0 phase shift from each other. Again, a reversed current through coils SC3 and SC4 having a 180xc2x0 phase shift from phase B is designated as phase {overscore (B)}. Motors with series connection work very well (maximize torque) for low speed applications.
FIG. 1D shows a parallel connection for the stator coils of FIGS. 1A and 1B. Here the stator coils SC1 and SC2 are connected in parallel at first ends a1 and a2 and also at second ends a1xe2x80x2 and a2xe2x80x2, the opposite ends forming terminals connected to phase A of a two-phase driver. Similarly, stator coils SC3 and SC4 are connected in parallel at first ends b1 and b2, and also at second ends b1xe2x80x2 and b2xe2x80x2, to form terminals connected to phase B of the two-phase driver, where phase B is a shifted 90xc2x0 relative to phase A. Again, phases {overscore (A)} and {overscore (B)} designate a reversed current through the coils which are 180xc2x0 out of phase relative to phases A and B, respectively. Motors with parallel connection work very well for high speed applications.
Neither the series connection nor the parallel connection of the stator coils work especially well (in terms of maximizing torque for a given rated current) at medium speed. Medium speed applications include surveillance cameras, printers, scanners, x-y tables, turn tables, CNC machines, dispensers, injector pumps, optical equipment and other precision devices. Many of these applications require smooth motion. Thus, in addition to torque, another relevant motor performance factor is motor noise or vibration, which is related to the magnetic flux path created by the stator coil and driver connections. It is desired to minimize vibration at operating speed of a two-phase motor, as well as maximizing torque at that speed. Medium speed refers to the pulse rate or speed of step motor operation at which neither series nor parallel connection performs significantly better than the other.
This objective of improved medium speed operation for two-phase step motors is achieved by a new stator coil and driver connection scheme, hereinafter called a T-connection. Unlike the prior connections in which the paired coils wound around the same group of poles were the ones connected together in series or parallel, in the present invention it is the coils wound around different poles that are connected together. The two stator coils in each set are connected in series, but in different ways for the two sets. In one set, both coils are connected in a forward sense around the stator, i.e. so current flows from pole to pole around the stator in the same direction for each of the two coils. In the other set, one of the coils is connected in series to the other in a reverse sense, i.e. so that the current flows from pole to pole around the stator in opposite directions for the two coils. The two sets of connected coils have terminals that connect to respective phases a and of a two-phase driver, where phase xcex1 and xcex2 are shifted 90xc2x0 relative to each other. The properties of this T-connection are found to be intermediate between that of the prior series and parallel connections and are best suited of the three types to medium speed motor operation.